Radiation is commonly used in the non-invasive inspection of objects such as luggage, bags, briefcases, and the like, to identify hidden contraband at airports and public buildings. The contraband may include hidden guns, knives, explosive devices and illegal drugs, for example.
FIG. 1 is a front view of one common X-ray scanning system 10, referred to as a line scanner. The object 12 to be inspected is conveyed through a shielded tunnel 13 between a stationary source of radiation 14, such as X-ray radiation, and a stationary detector array 16, by a conveying system 18. The radiation is collimated into a fan beam 20. Windows 21a, 21b are provided in the walls of the tunnel 13 to allow for the passage of radiation to the object 12 from the source 14 and from the object 14 to the detector array 16. The detector array 16 may also be provided within the shielded tunnel 13, in which case only one window 21a would be required. The conveyor system 18 may comprise a mechanically driven belt comprising material that causes low attenuation of the radiation. The conveyor system 18 can also comprise mechanically driven rollers, with gaps in the rollers to allow for the passage of the radiation. Shielding walls 22 surround the source 14, the detector 16 and a portion of the conveying system 18. Openings (not shown) are provided in the shielding walls 22 for the object to be conveyed into and out of the scanning system 10 by the conveying system 18. A second stationary source (not shown) may be provided above the conveying system 18 and a second stationary detector (not shown) may be provided below the conveying system (or vice-a-versa), to examine the object 10 from another angle.
Radiation transmitted through the object 12 is attenuated to varying degrees by the object and its contents. The attenuation of the radiation is a function of the density and atomic composition of the materials through which the radiation beam passes. The attenuated radiation is detected and radiographic images of the contents of the object 12 are generated for inspection. The images show the shape, size and varying densities of the contents.
The source 14 is typically a source of X-ray radiation of about 160 KeV to about 450 KeV. The X-ray source 14 in this energy range may be an X-ray tube. As shown in FIG. 1, the X-ray source 14 must be displaced a sufficient distance from the object 12 so that the fan beam 20 intercepts entire object. The fan angle 74 may be from about 30 degrees to about 90 degrees, for example. X-ray scanning systems, such as the system 10, are generally large.
X-ray radiation of 450 KeV will not completely penetrate large objects such as cargo containers. Standard cargo containers are typically 20–50 feet long (6.1–15.2 meters), 8 feet high (2.4 meters) and 6–9 feet wide (1.8–2.7 meters). Air cargo containers, which are used to contain plural pieces of luggage stored in the body of an airplane, may range in size from about 35×21×21 inches (0.89×0.53×0.53 meters) up to about 240×96×118 inches (6.1×2.4×3.0 meters). In contrast, typical airport scanning systems for carry-on bags have tunnel entrances up to about 0.40×0.60 meters. Only bags that fit through the tunnel may be inspected. Scanning systems for checked luggage have tunnel openings that are only slightly larger. Large collections of objects, such as many pieces of luggage, may also be supported on a pallet. Pallets, which may have supporting side walls, may be of comparable sizes as cargo containers. The low energies used in typical X-ray luggage and bag scanners, described above, are too low to penetrate through the much larger cargo containers or collections of objects. In addition, many such systems are too slow to economically inspect larger objects, such as cargo containers.
To inspect larger cargo containers, X-ray radiation of at least about 1 MeV range is required. Linear accelerators may be used to generate X-ray radiation in the MeV range. Linear accelerators are long (about 12–18 inches). In addition, the intensity of the radiation is greatest in a forward direction, along the longitudinal axis of the electron beam. The uniformity of the emitted radiation decreases as the angle from the forward direction is increased. To maintain beam uniformity, at average energy distortions of about 9 MeV, for example, narrow beams having an arc up to about 30 degrees tend to be used. With average energy distributions of about 3 MeV, beams having an arc up to about 65 degrees may be used. The smaller the arc, the farther the source must be in order to intercept the entire object. The length of the high energy X-ray sources and the beam arc tend to make higher energy X-ray scanning systems large. Since the space occupied by an X-ray scanning system could often be used for other important purposes, a more compact X-ray scanning system would be advantageous.
FIG. 2 is a schematic axial sectional view of an example of a prior art charged particle standing wave accelerator structure 50, referred to as a linear accelerator. The linear accelerator 50 comprises a chain of electromagnetically coupled, doughnut shaped resonant cavities 52, 54, with aligned central beam apertures 56. An electron gun 57 at one end of the chain of cavities emits an electron beam 57 through the apertures 56. A target 60 of tungsten, for example, is provided at an opposite end of the cavities 52, 54. The cavities 52, 54 are electromagnetically coupled together through a “side” or “coupling” cavity 61 that is coupled to each of the adjacent pair of cavities by an iris 62. The cavities are under vacuum.
Microwave power enters one of the cavities along the chain, through an iris 66 to accelerate the electron beam. The linear accelerator is excited by microwave power at a frequency near its resonant frequency, between about 1000 to about 10,000 MHz, for example. After being accelerated, the electron beam 58 strikes the target 60, causing the emission of X-ray radiation.
Movable plungers or probes 68 extend radially into one of the coupling cavities 70. One probe 68 is shown in FIG. 2. A corresponding probe is provided in the cavity 70 behind the probe 68 and cannot be seen in this view. The probes are moved under the control of a computer program to alter the magnetic fields within the cavity, to vary the energy of the accelerating electrons. The energy of the radiation generated by the electrons as the electron beam 57 impact the target is thereby varied. Such a linear accelerator 50 is described in more detail in U.S. Pat. No. 6,366,021 B1, which is assigned to the assignee of the present invention and is incorporated by reference, herein. Linear accelerators are also described in U.S. Pat. Nos. 4,400,650 and 4,382,208, which are also assigned to the assignee of the present invention and are incorporated by reference, herein.